1. Field of the Invention
The present invention relates to an optical scanning apparatus that is suitable for an image forming apparatus, such as a laser beam printer (LBP), a digital copying machine and a multi-function printer (MFP).
2. Description of the Related Art
To reduce the number of components, simplify the configuration and reduce the size, conventional optical scanning apparatuses include an optical system in which light is incident on a polygon mirror in a direction oblique to a sub-scanning direction (referred to as an oblique incident optical system).
To further reduce the size and the cost, various optical scanning apparatuses have been proposed in which imaging lenses close to photosensitive drums, which would typically be provided separately for the respective photosensitive drums, are replaced with a common imaging lens is shared among the photosensitive drums.
More specifically, a common imaging lens is shared among a plurality of photosensitive drums by providing a plurality of optical functional portions arranged in the sub-scanning direction, each optical functional portion having an incident optical surface and an exit optical surface for a light flux directed to a corresponding photosensitive drum.
Japanese Patent Application Laid-Open No. 2004-294713 and Japanese Patent Application Laid-Open No. 2008-15139 disclose optical scanning apparatuses that comprise an imaging lens including a plurality of optical functional portions arranged in the sub-scanning direction. However, the conventional techniques disclosed in Japanese Patent Application Laid-Open No. 2004-294713 and Japanese Patent Application Laid-Open No. 2008-15139 have the problems described below.
In Japanese Patent Application Laid-Open No. 2004-294713, it is described that there is a discontinuous part between optical functional portions arranged side by side in the sub-scanning direction in a multi-layer toric lens used as an imaging lens. It is also described that light fluxes having sub-scanning oblique incident angles whose signs are the same but whose absolute values are different are incident on the multi-layer toric lens. If optical functional portions designed for the light fluxes having sub-scanning oblique incident angles whose signs are the same but whose absolute values are different are arranged side by side in the sub-scanning direction in an imaging lens, a sag in the optical axis direction occurs at the discontinuous parts between the optical functional portions. Therefore, the transferability of shapes in molding deteriorates at the discontinuous parts, and as a result, the optical capabilities of the imaging lens can deteriorate. To avoid this, the incident surfaces and/or exit surfaces of the optical functional portions can be smoothly connected to each other by a spline curve surface so as to eliminate the sag in the optical axis direction. In that case, however, the optically effective area of the optical functional portions decreases, and as a result, required imaging properties may not be able to be achieved if the light fluxes are incident on the multi-layer toric lens at positions displaced in the sub-scanning direction due to a tolerance and the like. To avoid this, the sag in the optical axis direction at the discontinuous parts between the optical functional portions needs to be reduced.
The optical scanning apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-15139 uses a multi-layer toric lens including optical functional portions arranged side by side in the sub-scanning direction, the optical functional portions being designed for light fluxes having sub-scanning oblique incident angles whose absolute values are the same but whose signs are different. In the disclosed optical scanning apparatus, the incident surfaces and/or exit surfaces of the optical functional portions are connected to each other by a spline curve surface at the boundaries therebetween. However, the optical scanning apparatus according to Japanese Patent Application Laid-Open No. 2008-15139 does not take into consideration that light fluxes having sub-scanning oblique incident angles whose absolute values are different but whose signs are the same are each close to a boundary between optical functional portions in the sub-scanning direction. Therefore, with the optical scanning apparatus according to Japanese Patent Application Laid-Open No. 2008-15139, light fluxes having sub-scanning oblique incident angles whose absolute values are different but whose signs are the same can be displaced in the sub-scanning direction due to a tolerance and the like and therefore pass through the spline curve surface, thereby causing deterioration of the optical capabilities.
The present invention provides an optical scanning apparatus that comprises an imaging lens that adopts an oblique incident optical system and includes a plurality of optical functional portions arranged side by side in a sub-scanning direction and can provide an image of high quality without causing deterioration of optical capabilities by reducing a sag in an optical axis direction at a connecting portion between optical functional portions.